Videodiscs are molded records which have a signal track formed in the surface thereof which contains signal information which corresponds to a television broadcast signal.
U.S. Pat. No. 3,842,194, issued on Oct. 15, 1972 to J. K. Clemens, discloses a videodisc playback system of the capacitance electronic type. In one configuration of the Clemen's type videodisc, signal information which corresponds to video and audio programming is encoded in the form of a surface relief pattern in a relatively narrow spiral information groove molded into the surface of the videodisc made of a conductive plastic. During playback, the pickup stylus tracks in the spiral information groove as the videodisc is rotated by the turntable of the videodisc player. Capacitance variations between the electrode on the stylus and the conductive videodisc surface are detected and used to reconstruct the information recorded in the videodisc.
U.S. Pat. No. 4,162,510, issued on July 24, 1979 to E. O. Keizer, discloses a keel tip stylus for use with a capacitive electronic videodisc player of the type disclosed in the Clemens' patent. The stylus disclosed in the Keizer patent has a support of a hard wearing dielectric material such as diamond. The Keizer stylus has the conductive electrode formed on one surface thereof for detecting the variations in capacitance. The Keizer stylus is generally of a elongated rectangular configuration. The terminal end of the stylus which tracks in the information track of the videodisc, however, is shaped to a point so as to have a prow like front to guide the stylus in the information track. The terminal end is further shaped so as to have a keel-like section of reduced size to facilitate tracking of the stylus in the information groove. The keel is of an extremely small size, being about two microns in height and width and about 3-5 microns in length.
The formation of the keels on styli so as to have the required dimensions is one of the more difficult tasks involved in the manufacture of styli. In U.S. Pat. No. 4,104,832, issued on Aug. 8, 1972 to E. O. Keizer, there is disclosed a method in which a lapping disc having an abrasive on the surface thereof is used to lap the keel onto previously pointed styli. The lapping disc which is disclosed is similar to a molded videodisc except it has a groove formed in its surface which mates to the shape of the keel to be formed on the styli. An abrasive such as SiO.sub.x is applied to the surface of the lapping disc and previously pointed styli are guided through the groove until a sufficient material is lapped from the end of the styli to form keels of the desired configurations.
The method disclosed in the Keizer U.S. Pat. No. 4,104,832 has proven to be a highly successful method for forming a keel on a styli. Problems, however, are still encountered if the grooves of the lapping disc do not have the correct configuration along the entire length of the groove in the lapping disc. It is important that the width of the groove and its shape be uniform along its entire length. If the groove narrows, for example, it can cause excessive abrasion of the stylus being lapped or can inpart a dynamic shock to a stylus being processed so as to break the keel off the stylus. Other problems are likewise encountered as a result of defects in the lapping disc. Many of the defects in the lapping can be traced back to the matrixing process used in the manufacture of the disc.
The method used to manufacture lapping discs is in many respects similar to the method used to manufacture videodiscs; however, considerably more problems are encountered in the matrixing process when manufacturing lapping discs as compared to videodiscs. As disclosed in copending U.S. application 238,805 filed Feb. 27, 1981 by G. Kaganowicz, a groove having the desired shape of the keel is electromechanically recorded into a metal substrate. In the next step, the surface of the recorded metal substrate is passivated and thereafter a metal, typically nickel, is electroformed on the surface of the recorded substrate to form a master. After the master is electroformed to the desired thickness, typically 9 mils, it must be separated from the surface of the recorded substrate. It is at this point in the matrixing process used for lapping discs that considerable problems are encountered as compared to the matrixing processes used for videodiscs. With videodiscs once the outer edges and the center portions of the recorded substrate and the master are separated, it is generally relatively simple to separate the parts from each other. In the manufacture of lapping discs, however, even though the outer edges and center portions are well separated, it is often still extremely difficult, if not impossible, to separate the lapping disc master from the recorded substrate. Considerable mechanical force is often required, such as using metal clamps or the like to pull the master from the surface of the recording substrate. Very often the force required to separate the parts irrepairably damages either the recording substrate, the master or both. The reason for the greater difficulty encountered in separating the master from the recording substrate when making lapping disc is not known for certain.
The mechanical force involved in the techniques heretofore employed for the separation of the electroformed master from the recorded substrate is believed to be one of the major causes of distortion of the grooves molded into the lapping disc. There are protrusions on the lapping disc master which correspond to the grooves in the molded lapping disc that can readily be bent or crushed during separation so as to change the width of the lapping groove ultimately molded into the lapping disc.
After the lapping master is separated from the recorded substrate, it is in turn passivated and replicated by electroforming a metal again, such as nickel, on its surface to form what is referred to as a mold. Difficulties are also often encountered in separating the molds from the masters, but in general it is not as difficult as separating the lapping disc masters from the recorded substrate.
The matrixing process is completed by electroforming a metal such as nickel on the molds to form parts referred to as stampers. Difficulty is also encountered when separating stampers from the molds but again, not as much difficulty as when attempting to separate the master from the recorded substrate.
The stamper is the metal part which is actually used to mold the lapping disc from a plastic material such as a vinyl resin. In order to have a satisfactory stamper for use in the molding of the plastic lapping disc, it is important that the width and shape of the lapping grooves be precisely maintained in the desired configuration throughout the entire matrixing process.
What would be highly advantageous would be an improvement in the matrixing method used for manufacture of the lapping disc which will facilitate the separation of the electroformed parts from the parts on which they are electroformed and in particular an improved method to separate electroformed masters from metal recording substrates.